We have found significant racial and sex differences in calcium retention and kinetics in adolescents. American black girls had greater calcium retention than white girls (481 vs. 282 mg/d) on current recommended calcium intake as a result of greater net bone formation rates, higher calcium absorption and lower urinary calcium excretion. We also found white boys retain more calcium than girls at all calcium intakes. From these balance and kinetic studies, we have developed a model of the relationship between calcium intake and calcium retention that can be used to estimate calcium requirements for maximal skeletal accretion in adolescents. We are currently studying these relationships in Chinese American adolescents. Chinese boys and girls have greater calcium retention at all calcium intakes than white boys and girls. In this application, we propose to study these relationships in adolescent boys and girls from the Mexican American minority who are the largest group of Hispanics and will soon comprise one-fourth of the U.S. population. We hypothesize that Mexican Americans respond to calcium intake like other minority groups with increased calcium absorption efficiency at low calcium intakes in comparison to white adolescents. The first aim is to establish the calcium requirement in Mexican American girls and boys;the second aim is to establish the relationship between calcium intake and fractional calcium absorption;the third aim is to determine the relative contribution of the intestine, kidney, and bone to calcium retention in Mexican American adolescents and compare these results with those already established in American white, black, and Asian adolescents;and the fourth aim is to develop a model to quantify the effect of calcium intake, race/ethnic group, sex, and sexual maturity on calcium retention. As we have done in our previous studies, we will use stable calcium isotope kinetics coupled with metabolic balance studies conducted as a summer research camp. Optimizing calcium retention during adolescence in both majority and minority populations in the U.S. is a key nutritional strategy to build peak bone mass and reduce lifelong risk of osteoporosis in the elderly U.S. population. PUBLIC HEALTH RELEVANCE: Hispanics will soon comprise one-fourth of the U.S. population. This project will determine the role of dietary calcium in skeletal calcium retention in Mexican American adolescents and will compare these results with what we have found in other racial groups to identify mechanisms responsible for the racial differences. These studies will allow us to determine the calcium intake required for optimal calcium retention during the period of rapid skeletal growth which is a key nutritional strategy to build peak bone mass and reduce lifelong risk of osteoporosis.